For the LTE-Advanced radio technology which is the further advancement of Long Term Evolution, discussions and research are being actively conducted on heterogeneous networks. A heterogeneous network is a network in which mutual connection is established under the circumstances where cells and/or communication devices with different operating systems, protocols, or accessing technologies coexist. For example, mini base stations such as Home eNode Bs (HeNBs), also known as femtocell base stations, are placed and coexist with macro cell base stations (MeNBs) with different transmit power levels or different purposes of operations, and telecommunications are carried out between heterogeneous cells and/or within each of the heterogeneous cells.
A closed subscriber group (CSG) cell is also possible, in the context of heterogeneous networks. A CSG cell provides services only to a limited number of user equipment (UE) terminals that are allowed to access the HeNB.
If a MeNB and HeNBs operate at the same carrier frequency in a MeNB-HeNB coexisting environment, serious issues of mutual interference will arise. There are two possible scenarios of interference.
FIG. 1A illustrates the first scenario, in which a user equipment (MUE) 11 is located in the vicinity of a HeNB 20 and connected to a macro cell (MeNB) 10 on the uplink, creating a large amount of uplink interference in the HeNB 20. To be more precise, if the MUE 11 is accessing the MeNB 10 using the serving link, while the HeNB 20 is receiving signals from a Home eUE (HeUE) 21 which is operating as a CSG member with access to the HeNB in the CSG cell, the radio communication waves transmitted from the MUE 11 become significant interference to the HeNB 20, as indicated by the dashed arrow.
FIG. 1B illustrates the second scenario, in which the HeNB 20 is currently transmitting signals to the HeUE 21 in the vicinity of the MUE 11 and creating a large amount of downlink interference to the MUE 11. To be more precise, if the MUE 11 is receiving signals from the macro cell MeNB 10 during the signal transmission from the HeNB 20 to the HeUE 21, the radio communication waves transmitted from the HeNB 20 to the HeUE 21 become considerable interference to the MUE 11, as indicated by the dashed arrow.
These interference issues are likely to arise not only when the MUE 11 is a non-CSG member of the HeNB 20, but also when the MUE 11 is a CSG member of the HeNB 20, if the MUE 11 is connected to the macro cell MeNB10 in the vicinity of the HeNB 20. A similar problem will occur when the HeNB 20 is located at the edge or the boundary of the macro cell.
Several techniques of intercell interference coordination have been proposed; however, intercell interference cannot be controlled efficiently with these techniques in a heterogeneous network because of their own limits in performing interference coordination. One known technique is to report frequency resources at high interfering levels to neighboring cells by X2 signaling (i.e., notification of overload indicators between macro cells through a wired channel). See, for example, 3GPP TS 36.423. However, this technique can hardly identify which user equipment (MUE) is the interference source in the heterogeneous network because the notification pattern does not have enough granularity in the time domain.
Another known technique is to make use of a measurement report which is adapted to be transmitted from a UE to the macro cell base station when the UE finds a neighboring cell at a high signal level. See, for example, 3GPP TS 36.331. However, transmission of the measurement report is triggered only by a specific event and is not constantly transmitted. Furthermore, additional problems, in that radio resources are occupied by the UE when reporting the measurement result to the macro cell and that the UE bears a great burden, will arise.